In recent years, algae have received attention as a raw material for supplements such as EPA and DHA. In addition, there is a growing interest also in the algae as food. Furthermore, it is studied to use the algae as biofuel as alternative energy to fossil fuels. Thus, a demand for the algae is expected to increase, and the constructions of production plants for the algae are also proceeding both in Japan and abroad. From 2020, EU and other countries require to mix 20% of biofuel into a jet fuel. However, the jet fuel purified from cultured algae water has a problem of increased production costs if the jet fuel is produced by a current algae water concentration method, and the jet fuel is inferior to the fossil fuels in terms of the cost.
There are many types of algae, and there are also various sizes. Nannochloropsis is 5 μm and Botryococcus and Aurantiochytrium are approximately 50 μm. Thus, the size greatly varies depending on the type.
In the algae water plant, algae are cultured in a culture pond; and large-grown algae are collected, and are sent to the next step of drying and oil extraction. However, the algae which have been collected from the culture pond together with water contain algae having a size that does not satisfy the specification, and there is too much moisture for use in the drying and oil extraction step. Accordingly, it is necessary to concentrate the algae water while sorting the algae which have been cultured in the culture pond. Currently, it is a common practice to perform algae water concentration by a method of separating and concentrating the algae water with a centrifugal separator or a method of separating and concentrating the algae and the water with a flat membrane, a hollow fiber membrane or a reverse osmosis membrane. (See Patent Literature 1, for instance).
In the case of the separation and concentration method by the centrifugal separator, the amount of capital investment is large, and the facility maintenance cost and the operation cost increase. In addition, in the case of the separation and concentration method using the flat membrane, the hollow fiber membrane or the reverse osmosis membrane, it is necessary to apply high pressure to the separation and concentration. In addition, the membrane must be periodically cleaned (backwash) with high pressure water or gas, and high pressure equipment is needed such as a high pressure compressor. As a result, the amount of capital investment, the facility maintenance cost and the operation cost increase.
In addition, in the case of the above described hollow fiber membrane and the reverse osmosis membrane, the membranes may capture all the cultured algae having all the sizes because of the characteristics of the opening structure of the membranes; it becomes difficult to separate the algae of a size that does not satisfy the specification, and discharge the separated algae to a culture pond again to culture the algae; and there is a problem of reduced production efficiency of algae culture.
Furthermore, it is known that the algae secrete mucus (see Non Patent Literature 1, for instance). When the algae secrete mucus, there is a concern that the mucus gets entangled with a filter to cause clogging of the filter. Then, it is desirable to previously remove the mucus from the algae in the algae water which is concentrated by the algae water concentration system. The document concerning the mucus of the algae has not been found, but there is a detailed report concerning the mucus of coral (see Non Patent Literature 2, for instance).
It is said that the coral secretes the mucus for self-defense. The self-defense means the defense against the growth of organisms and bacteria which have adhered to the surface of the coral, the defense from ultraviolet rays, the defense from pollutants, the defense from drying at the time when the coral has been exposed to air at low tide, and the like. In addition, it is considered that the coral secretes the mucus also because of predation or in connection with photosynthesis of zooxanthellae that live symbiotically in the coral.
The present inventors have come to an idea that the present inventors can provide an algae water concentration system which prevents mucus from getting entangled with a filter to cause the clogging of the filter, by giving the algae an environment that the coral does not secrete the mucus, on the basis of an assumption that the algae secretes the mucus according to a similar reason to the coral.